A heat pump is a device for moving heat from a low temperature reservoir to a high temperature reservoir. A heat pump which may be used to heat or cool an indoor space by pumping a refrigerant around a closed loop, includes a compressor, indoor and outdoor heat exchange coils, refrigeration piping, a refrigerant flow reversing valve and expansion devices and check valves. The change from the cooling function to the heating function, and vice versa, is achieved by reversing the direction of refrigerant flow in the system. Because the design mass flow rate for the cooling mode may be from 10% to 80% greater than for the heating mode, two expansion devices having different sizes are used in a typical heat pump system. In each mode, the refrigerant bypasses that expansion device which is to be passive and is metered through the active expansion device.
An expansion device which is capable of combining the functions of both expansion devices and associated check valves could result in reduced costs of hardware and installation, as well as operational advantages, and attempts have been made in the prior art to replace the two expansion devices with one. However, the known resulting devices have been both expensive and complex.
For example, U.S. Pat. No. 3,444,699 to Harnish is concerned with a heat pump in which a single expansion valve is used which responds to the rate of flow of refrigerant liquid by taking advantage of the cooling effect of the liquid to activate a valve piston towards either a more closed or more open position. U.S. Pat. No. 4,548,047 to Hayashi et al is concerned with a short tube restrictor as an expansion device in which a plunger opens and closes valve ports for changing the rate of refrigerant flow.
These and other known expansion devices increase the complexity of heat pumps instead of simplifying them.